It is known that dissolved substances can be separated from their solvents by the use of selective membranes. For example, of great practical interest is the removal of salt from water by reverse osmosis. The efficiency and economy of such removal is of tremendous economic significance in order to provide potable water from brackish or sea water for household or agricultural use. A critical factor in desalination is the performance of the membrane in terms of salt rejection, i.e., the reduction in salt concentration across the membrane, and flux, i.e., the flow rate across the membrane. For practical applications, the flux should be on the order of greater than about 10 gallons/ft.sup.2 -day (gfd) at a pressure of about 55 atmospheres for sea water and about 15 gfd at a pressure of about 15 atmospheres for brackish water. More preferably, commercial applications now require fluxes greater than about 25 gfd (about 1.0 m.sup.3 /m.sup.2 -day) at a pressure of about 15 atmospheres for brackish water. Moreover, salt rejections greater than 99% are required. The continuing goal of research and development in this area is to develop membranes having increased flux and/or salt rejection which are useful in desalination.
Among the known membranes used in desalination are included a large number of various types of polyamides which are prepared by a variety of methods. Of particular interest within this broad group of polyamide membranes are crosslinked aromatic polyamide membranes. The crosslinked aromatic polyamide membranes include, for example, those disclosed in the following U.S. Patents.
U.S. Pat. No. 3,904,519, issued to McKinney et al., discloses reverse osmosis membranes of improved flux prepared by crosslinking aromatic polyamide membranes using crosslinking agents and/or irradiation. The polyamides are prepared, for example, by the interfacial polymerization of amine groups and carboxyl groups followed by crosslinking.
U.S. Pat. No. 3,996,318, issued to van Heuven, teaches the production of aromatic polyamide membranes, wherein crosslinking is achieved using a reactant having a functionality of three or greater.
U.S. Pat. No. 4,277,344, issued to Cadotte, describes a reverse osmosis membrane which is the interfacial reaction product of an aromatic polyamine having at least two primary amine substituents with an aromatic acyl halide having at least three acyl halide substituents. The preferred membrane is made of a poly(phenylenediamine trimesamide) film on a porous polysulfone support.
U.S. Pat. No. 4,828,708, issued to Bray, discloses a similar membrane in which a major portion of the trifunctional aromatic acyl halide is replaced by the difunctional aromatic acyl halide-isophthaloyl chloride.
U.S. Pat. No. 4,529,646, issued to Sundet, shows a membrane similar to U.S. Pat. No. 4,277,344 in which all or a portion of the trifunctional aromatic acyl halide is replaced by cyclohexane-1,3,5-tricarbonyl chloride. Similar membranes are disclosed in U.S. Pat. Nos. 4,520,044; 4,544,484; 4,626,468; 4,643,829; and 4,783,346, each issued to Sundet (du Pont).
U.S. Pat. No. 4,761,234, issued to Uemura et al., shows a membrane similar to U.S. Patent 4,277,344 in which aromatic tri- or higher aromatic amines are employed
U.S. Pat. No. 4,661,254, issued to Zupanic et al., discloses a reverse osmosis composite membrane formed by the interfacial polymerization of a triaryl triamine with an aromatic carboxylic acid chloride.
U.S. Pat. No. 4,619,767, issued to Kamiyama et al., describes membranes prepared by crosslinking polyvinyl alcohol and secondary di- or higher amines with polyfunctional crosslinking agents. Both aromatic and aliphatic amine components are disclosed.
U.S. Pat. No. 4,749,488, issued to Arthur et al., discloses membranes of polyphenylene tetrahydrofuran-2,3,4,5-tetracarboxamide which may also include isophthalamide or terephthalamide units. Interestingly, Examples 19 and 20 of this patent describe preparation of a membrane prepared by reacting all-cis cyclopentane-1,2,3,4-tetracarbonyl tetrachloride (CPTC) with metaphenylene diamine (MPD) which showed totally unacceptable salt rejection results (Examples 21 and 22) compared to the membranes claimed in the patent.
U.S. Pat. Nos. 4,872,984 and 4,948,507, issued to Tomaschke and assigned to the same assignee as the present application, describe the interfacial synthesis of reverse osmosis membranes from an essentially monomeric polyamine having at least two amine functional groups and an essentially monomeric polyfunctional acyl halide having at least about 2.2 acyl halide groups per reactant molecule, in the presence of a monomeric amine salt. Both aromatic and aliphatic polyamines and polyfunctional acyl halides are disclosed.
Interesting reviews and comparisons of various composite reverse osmosis membranes are included in J. E. Cadotte, "Evolution of Composite Reverse Osmosis Membranes," Materials Science of Synthetic Membranes Chapter 12, pp. 273-294, American Chemical Society Symposium Series (1985) and S. D. Arthur, "Structure-Property Relationship in a Thin Film Composite Reverse Osmosis Membrane," Journal of Membrane Science, 46:243-260, Elsevier (1989).
While some of the above-referenced membranes are commercially useable, the goal of the industry continues to be to develop membranes that have better flux and salt rejection characteristics in order to reduce costs and increase efficiency of operation.